The invention relates to a method of imaging the blood flow as a function of time in an object to be examined, as well as to an X-ray device for carrying out this method.
EP 860 696 A2 discloses a method which enables the reproduction of distributed structures, for example a vascular system filled with a contrast medium, in a synthetic projection image which reproduces the parts of the vascular system that are present in a selectable sub-volume more clearly than the X-ray images that are acquired from different perspectives and wherefrom this projection image is derived. This method thus yields a time-averaged xe2x80x9cfrozenxe2x80x9d image of the vascular system in which the blood flow as a function of time is not visible.
Three-dimensional rotation angiography produces a series of X-ray projection images of the object to be examined from different projection directions while a contrast medium is injected into the blood vessels of the object to be examined. Using a known reconstruction algorithm, for example the Feldkamp algorithm, a three-dimensional image is derived from such X-ray projection images; this three-dimensional image reproduces the vascular system in space in a time-averaged manner. In two-dimensional angiography it is also known to form a two-dimensional xe2x80x9cfrozenxe2x80x9d image of the vascular system.
For various applications, such as the analysis of pathologies of the cerebral vessels, for example in the case of vascular anomalies (stenoses, arteriovenous deformations), however, it is important to know and reproduce the blood flow as a function of time. Therefore, it is an object of the invention to provide a method which enables the reproduction of the blood flow in an object to be examined as a function of time, and to provide an appropriate X-ray device for carrying out this method.
Such a method should also take into account the fact that a contrast medium cannot be repeatedly injected into the same patient within a short period of time, so that the method should do, if at all possible, with a single contrast medium injection. Moreover, it should be possible to acquire the images in an as short as possible period of time, at an as small as possible expenditure and with an as high as possible resolution.
This object is achieved by means of the method disclosed in claim 1 and the X-ray device disclosed in claim 10.
The invention is based on the recognition of the fact that an image data set, which may be a two-dimensional or a three-dimensional image data set and contains information concerning the course of the blood vessels in the object to be examined, can be encoded in time in such a manner that it also contains information concerning the blood flow as a function of time. Such encoding in time is performed according to the invention in that the image data set is compared with a series of X-ray projection images; these X-ray projection images are formed successively in time and contain the information concerning the distribution of an injected contrast medium in the blood vessels at each time a different instant. Because each X-ray projection image is individually compared with the image data set, that is, each image value of the image data set is compared with the image values of the individual X-ray projection images, it is quasi checked which parts of the vascular system contained in the image data set are filled with the contrast medium at the individual instants associated with the respective X-ray projection images. Using suitable reproduction methods, the image data set thus encoded in time can be converted into one or more images which show the blood flow as a function of time.
The version disclosed in claim 2 is particularly suitable for two-dimensional rotation angiography. It already offers a two-dimensional X-ray image data set, for example a two-dimensional X-ray projection image which completely contains the vascular system filled with contrast medium and may be a previously formed or an instantaneous X-ray image data set. The actual X-ray projection images derived from a fixed X-ray position during a contrast medium administration are subtracted from one another in conformity with this version, so that each difference image contains the information as regards the path followed by the contrast medium in the vascular system between the two instants at which the two subtracted X-ray projection images were acquired. Such difference images are then used for the time encoding of the X-ray image data set.
A further version of this method is disclosed in claim 3. This version represents a simple possibility for comparing the image data set with the difference images. In order to enhance the imaging precision, first an X-ray image data sub-set is segmented from the X-ray image data set; this sub-set contains only the information concerning the course of the blood vessels. Subsequently, for each difference image there is acquired an associated pixel sub-set in such a manner that the pixels of the individual difference images are compared with the pixels of the X-ray image data sub-set and that with each pixel sub-set there are associated those pixels of the X-ray image data sub-set for which the associated difference image includes corresponding pixels. Each pixel sub-set thus contains the information concerning the distribution of the contrast medium at a given instant and one or more projection images which show the blood flow as a function of time can be derived from the pixel sub-sets.
According to the preferred version disclosed in claim 4, the X-ray projection images are acquired from different positions. The image data set then constitutes a three-dimensional X-ray image data set derived from such X-ray projection images. Thus, according to this further version only a single series of X-ray projection images is acquired from different directions, said images also containing the time information. After segmentation of the blood vessels in said X-ray projection images they are compared with the X-ray image data set and encoded in time, for example, in that pseudo-projection images are calculated from the X-ray image data set, utilizing the known imaging geometry of the X-ray device, so as to be compared with the actual X-ray projection images.
Claim 5 discloses a particularly attractive further version which is also suitable for three-dimensional rotation angiography. Two series of X-ray projection images are acquired therein, the first series being acquired from a fixed X-ray position whereas the second series is acquired from different X-ray positions. This operation can be performed either successively in time, be it that two contrast medium injections are then required, or simultaneously by means of an X-ray device which includes two imaging units. From the X-ray projection images of the first series there are derived difference images which, as described above, contain the time information whereas a three-dimensional X-ray image data set is acquired from the X-ray projection images of the first series by means of a known reconstruction algorithm. This X-ray image data set is encoded in time by means of the difference images. This further version, notably the formation of difference images as carriers of the time information, offers the advantage that the difference images always contain only the variation in time of the contrast medium flow (=the blood flow) in a given time interval whereas in the case of direct use of the X-ray projection images as carriers of the time information the overall distribution of the contrast medium in the vascular system is always contained therein and hence the information is significantly less exact. This is also due to the fact that the contrast medium propagates very quickly throughout the vascular system to be observed, so that the differences in the distribution of the contrast medium between two comparatively closely spaced instants are only comparatively small.
The claims 6 and 7 disclose preferred possibilities for the time encoding of the image data set and the comparison of the image data set with the segmented X-ray projection images. The steps of the method as disclosed in claim 7 correspond to the method which is known from EP 880 109 A2 whose disclosure is explicitly referred to herein and is considered to be incorporated herein. The latter publication describes a method of determining the spatial transformation between a three-dimensional object reproduced by a data set and the object itself. A pseudo-projection image is then calculated for a part of the volume reproduced by the data set; this pseudo-projection image is compared with an X-ray projection image of the object itself. The parameters on which the calculation of the pseudo-projection image is based are then varied until an optimum match is obtained. This method can be advantageously used in an appropriate manner in conjunction with the present invention.
The claims 8 and 9 disclose advantageous further versions concerning the display of the blood flow as a function of time. For example, the time information can be converted into a color code so that the complete two-dimensional or three-dimensional data set can be reproduced as an image with the corresponding color code. The encoded pixel sub-sets or voxel sub-sets can also be reproduced successively in time, for example as an endless loop, thus creating the impression of blood flowing through the blood vessels. It is also feasible to enable reproductions from arbitrary angles of observation and rotation of the image is also possible.
An X-ray device which is suitable for carrying out the method according to the invention is disclosed in claim 10; as is indicated in the embodiment of claim 11, it may also include a second imaging unit.
The invention will be described in detail hereinafter with reference to the drawings. Therein:
FIG. 1 shows an X-ray device for carrying out the method according to the invention,
FIG. 2 shows a flow chart illustrating a first version of the method according to the invention,
FIG. 3 illustrates diagrammatically the encoding in time,
FIG. 4 shows a flow chart of a second version of the method according to the invention, and
FIG. 5 shows a flow chart of a third version of the method according to the invention.